Two protocols for the conversion of biphenol to binaphthol: Synthesis of diospyrol

Article abstract of DOI:10.1055/s-2006-939075

The application of directed orthometallation (DoM), Fries rearrangement and transmetallation followed by allylation and cyclization is reported for the conversion of biphenol to binaphthol as a means for the synthesis of diospyrol. Furthermore, the same t

Full text of DOI:10.1055/s-2006-939075

1080
LETTER
Two Protocols for the Conversion of Biphenol to Binaphthol: Synthesis of
Diospyrol
Synthesisof
D
iosp
o
yrol pporn Thasana,a Somchai Pisutjaroenpong,a Somsak Ruchirawat*a,b
a
Chulabhorn Research Institute, Vipavadee Rangsit Highway, Bangkok 10210, Thailand
Fax +66(2)5740616; E-mail: somsak@cri.or.th
b
Programme on Research and Development of Synthetic Drugs, Institute of Science and Technology for Research and Development,
Mahidol University, Salaya Campus, Nakornpathom 73170, Thailand
Received 11 July 2005
binaphthol could be directly generated via aryne annula-
tion. It was anticipated that the trapping of bisaryne 8, de-
rived from tetrabromobiphenyl ether 6, with diene 9,
generated from unsaturated amide 7, could lead directly to
Abstract: The application of directed orthometallation (DoM),
Fries rearrangement and transmetallation followed by allylation and
cyclization is reported for the conversion of biphenol to binaphthol
as a means for the synthesis of diospyrol. Furthermore, the same
transformation can be accomplished by the reaction of the dienolate the binaphthol 2. An alternative mechanism for the forma-
anion of an a,b-unsaturated amide with an aryne intermediate.
tion of the same binaphthol 2 could involve the sequential
reactions of the monoaryne derived from compound 6
with diene 9 followed by the reaction of another
monoaryne and diene 9. The tetrabromobiphenyl ether 6
could be easily obtained from the same biphenol 5 as used
in the first pathway.
Key words: arynes, biaryls, ring closure, directed orthometallation,
Fries rearrangement
The regiospecific preparation and modification of
polysubstituted aromatic molecules has remained a funda-
mental problem in organic synthesis in both industrial and
academic laboratories.1,2 The directed orthometallation
(DoM) reaction, discovered 70 years ago by Gilman3 and
Wittig,4 has been extensively studied and exploited for the
regioselective construction of polysubstituted aromatics
and heteroaromatics.
OH
R
HN
R
OH
M
OH OH
OH OH
OH OMe
O
Diospyrol (1) has been isolated from Diospyros mollis, a
tree distributed throughout Thailand. It has a dimeric
naphthalene skeleton with a C-2–C-2¢ linkage between the
1-naphthol ring systems.5 The fresh berries of this plant
have long been used especially as anthelmintics.6 Recent-
ly, michellamine alkaloids have been isolated and report-
ed to exhibit potent anti-HIV activity.7 Their structures
are composed of two important units, a 1,3-dimethyltet-
rahydroisoquinoline and the core binaphthol. The struc-
ture of the core binaphthol is also similar to diospyrol.
Michellamine B (Figure 1), the most studied compound of
this group, showed interesting activity to protect MT-2
cells from both AZT-resistant and pyridone-resistant
strains of HIV-1.8 Several strategies have been evolved
for the construction of this unit.9,10
MeO
OH
HO
diospyrol (1)
P
R
NH
R
michellamine B
OH
Figure 1 Structure of diospyrol (1) and michellamine B
Herein, harnessing Snieckus chemistry, we report the
application of directed orthometallation (DoM),11 Fries
rearrangement,12 transmetallation–bis-allylation,13 and
double cyclization to the synthesis of binaphthol 2. We
also report the aryne cycloaddition reaction10 of dienolate
anion 9 of a,b-unsaturated amide 7 with tetrabromobi-
phenyl ether 6 in the presence of strong base affording
diospyrol derivative 2.
We envisaged two pathways for the synthesis of the
binaphthol, both starting from biphenol as shown in
Scheme 1. In route A, the binaphthol could be derived
from the double cyclization of the allyl carbanion onto the
adjacent carboxamide group in compound 3. Compound 3
could be synthesized from the o-allylation of carboxamide
4 which could be obtained from the biphenol 5. In the
second pathway as shown in route B it was planned that
Commercially available biphenol 5 was first reacted with
N,N-diethylcarbamoyl chloride and NaH in DMF to yield
dicarbamate 10 (75%) as shown in Scheme 1.11 N,N-Di-
ethyl-2,2¢-dihydroxy-1,1¢-biphenyl-3,3¢-dicarboxamide 4
(R = H) was obtained in good yield (80%) by double
anionic ortho-Fries rearrangement12 of 2,2¢-dicarbamate-
1,1¢-biphenyl 10 using t-BuLi and TMEDA.14 Compound
4 (R = H) was then protected as its methyl ether, iso-
propyl ether and methylenedioxy ether by reaction with
MeI, 2-bromopropane or dibromomethane to give N,N-
diethyl-2,2¢-dialkoxyl-1,1¢-biphenyl-3,3¢-dicarboxamides
SYNLETT 2006, No. 7, pp 1080–1084
0
3.
0
5.
2
0
0
6
Advanced online publication: 24.04.2006
DOI: 10.1055/s-2006-939075; Art ID: D19405ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Diospyrol
1081
Br
OH
NR1R2
OR
O
OR
OH OR
A
B
R2R1N
NEt2
Et2N
OR
OH
OR
O
OR OH
Br
3
2
9
8
9
Br
OR
Br
O
OR
OR
R2R1N
O
Br
NEt2
Br
Et2N
NR1R2
O
OR
OR
O
OR
4
5
7
6
7
Scheme 1 Synthetic plans for the conversion of biphenol to binaphthol.
4a–c in excellent yields (94–100%). These were in good yield (75%) when six equivalents of MeLi (entry
ortholithiated using t-BuLi/TMEDA in THF at –78 °C, 6)17 were used but in lower yield (67%) when only four
transmetallated with CuCN/LiCl13 and trapped with b- equivalents of MeLi were employed (entry 5). When the
methallyl chloride in a one-pot reaction.
MeLi-induced cyclization (6 equiv of MeLi) was also
applied to compound 3b, the required product 2b was
obtained in 52% (entry 7).
The reaction was allowed to warm to room temperature
and stirred overnight to give the required compounds
3a–c in moderate to good yields as shown in Scheme 2.
Compound 3c gave a complex mixture on treatment with
LDA and MeLi.16 Attempts to activate the carboxamide
group of compound 3a with Tf2O in the presence of
pyridine18 to induce cyclization also failed.
We have investigated various bases and conditions for the
base-induced double cyclization of compounds 3. When
compound 3a was treated with five or ten equivalents of
LDA in THF, complex mixtures of products were ob- The two-step double cyclization and methylation of the
tained. Treatment of compound 3b with five equivalents intermediate 3a to tetramethoxydiospyrol was also exam-
of LDA gave the desired product 2b in 21% yield together ined in a one-pot process and provided a good yield (75%)
with the half-cyclized product 11b in 33% yield (Table 1, of the product. The reaction was carried out using six
entry 3). Increasing the amount of LDA to ten equivalents equivalents MeLi for double cyclization and the crude
gave lower yields of both compounds 2b and 11b (entry product was used in the next step without purification by
4). It was gratifying to find that compound 3a could be in- methylation with MeI in presence of NaH in DMF. The
duced to cyclize to the corresponding binaphthol by using spectroscopic data of the compound obtained were iden-
MeLi.16 The desired binaphthol 2a (R = Me) was isolated tical with those derived from tetramethoxydiospyrol
O
OH
Et2N
O
a
b, c
OH
O
NEt2
O
5
10
O
OR
O
OR
NEt2
d, e, f
NEt2
Et2N
Et2N
OR
O
OR
O
4
3
a R = Me, 90%
b R = i-Pr, 46%
c R = -CH2-, 47%
Scheme 2 Reagents and conditions: a) ClCONEt2, NaH, DMF (75%); b) t-BuLi, TMEDA, THF, –78 °C (80%);14 c) RX, NaH, DMF (4a:
R = Me, 94%, 4b: R = i-Pr, 100%, 4c: R = -CH2-, 98%); d) t-BuLi, TMEDA, THF, –78 °C; e) CuCN, LiCl, THF; f) b-methallyl chloride (3a:
90% over 3 steps, 3b: 46% over 3 steps,15 3c: 47% over 3 steps).
Synlett 2006, No. 7, 1080–1084 © Thieme Stuttgart · New York

1082
N. Thasana et al.
LETTER
Table 1 Double Ring Closure of Key Intermediates 3a–c
OH OR
OH OR
conditions
NEt2
3a–c
+
THF, –78 °C
to r.t.
OR
OH
OR
O
2
11
Entry
R
Conditions
Yield of 2 (%)
Yield of 11 (%)
1
2
3
4
5
6
7
Me
Me
i-Pr
i-Pr
Me
Me
i-Pr
LDA (5 equiv)
LDA (10 equiv)
LDA (5 equiv)
LDA (10 equiv)
MeLi (4 equiv)
MeLi (6 equiv)17
MeLi (6 equiv)
Complex mixture
Complex mixture
21
19
67
75
52
33
29
–
–
–
synthesized by another route.9 The tetramethoxydiospyrol Having the tetrabromobiphenyl ether 6 in hand, the aryne
could be demethylated to diospyrol by the previously annulation was then investigated. N,N-Diethylsenecio
published procedure.9b,c
amide 7a was treated with an excess of LDA at –78 °C in
THF in order to generate the lithiated amide. The tetrabro-
mobiphenyl ether 6 was added to the solution to generate
the aryne and the mixture was allowed to warm to room
temperature. After purification, the undesired LDA addi-
tion products were obtained.
The remarkable regiospecificity of the aryne annulation
reaction10 has been extensively used for the synthesis of
naphthols. As an extension of this type of synthetically
useful cycloaddition reaction, we were interested in the
application of this approach for the synthesis of binaph-
thols in general and the synthesis of diospyrol in parti- To overcome this problem, the more hindered base, LT-
cular. With this idea in mind, biphenol 5 was converted to MP, was used. Treatment of tetrabromobiphenoldimethyl
tetrabromo-2,2¢-dihydroxybiphenyl by bromination with ether 6 with an excess of LTMP and N,N-diethylsenecio
bromine in AcOH in quantitative yield. The tetrabro- amide 7a gave dibromodiospyrol adduct 12 directly in
mophenol was methylated using dimethylsulfate and 20% yield together with other unidentified products.19
K2CO3 in refluxing acetone to give tetrabromo-2,2¢- Using the Weinreb amide 7b, a lower yield (14%) of
dimethoxybiphenyl 6 in 67% yield.
binaphthol 12 was obtained.
Methylation of 4,4¢-dibromodiospyrol 12 with dimethyl-
sulfate in the presence of K2CO3 in refluxing acetone gave
4,4¢-dibromodiospyrol tetramethyl ether 13 in high yield
(96%) which was converted to diospyrol tetramethyl ether
14 by debromination with tributyltin hydride in good yield
(81%).20 The dibromodiospyrol derivative was recently
synthesized by a different approach.21 Significantly, the
remaining bromine group can be used as a handle for
further coupling.
Br
OMe
Br
OH
c
a, b
Br
Br
O
NR1R2
OMe
OH
6
5
7a R1 = R2 = Et
7b R1 = OMe, R2 = Me
X
In summary, we have successfully developed two direct
approaches for the conversion of biphenol to binaphthol
and applied to the synthesis of diospyrol. The methodolo-
gy should be applicable to the synthesis of related oxygen
heterocycles.
Br
OMe OMe
OH OMe
d
OMe
OMe
e
OMe OH
X
Br
13 X = Br
14 X = H
12
Acknowledgment
Scheme 3 Reagents and conditions: a) Br2, AcOH; b) Me2SO4,
K2CO3, acetone, reflux (67%, 2 steps); c) LTMP, THF, –78 °C (20%
from 7a,19 14% from 7b); d) Me2SO4, K2CO3, acetone, reflux (96%);
e) n-Bu3SnH, AIBN, reflux (81%).
We acknowledge the Thailand Research Fund (TRF) for the ge-
nerous support of our research programme and the award of Senior
Research Scholar to SR as well as the award of a research grant
(TRG4680008) to N.T.
Synlett 2006, No. 7, 1080–1084 © Thieme Stuttgart · New York

LETTER
Synthesis of Diospyrol
1083
(14) Directed Orthometallation (DoM) and Fries
References and Notes
Rearrangement.
(1) (a) Snieckus, V. Chem. Rev. 1990, 90, 879. (b) Green, L.;
Chauder, B.; Snieckus, V. J. Heterocycl. Chem. 1999, 36,
1453. (c) Chauder, B.; Green, L.; Snieckus, V. Pure Appl.
Chem. 1999, 71, 1521. (d) Whisler, M. C.; MacNeil, S.;
Snieckus, V.; Beak, P. Angew. Chem. Int. Ed. 2004, 43,
2006.
(2) Clayden, J. Organolithiums: Selectivity for Synthesis, Vol.
23; Baldwin, J. E.; Williams, R. M., Eds.; Tetrahedron
Organic Chemistry Series, Pergamon Press: Oxford, 2002.
(3) Gilman, H.; Bebb, R. L. J. Am. Chem. Soc. 1939, 61, 109.
(4) Wittig, G.; Fuhrmann, G. Chem. Ber. 1940, 73, 1197.
(5) (a) Loder, J. W.; Mongkolsuk, S.; Robertson, A.; Whalley,
W. B. J. Chem. Soc. 1957, 2233. (b) Mongkolsuk, S.;
Sdarwonvivat, C. J. Chem. Soc. 1965, 1533. (c) Yoshihira,
K.; Natori, S.; Kanchanapee, P. Tetrahedron Lett. 1967,
4857. (d) Yoshihira, K.; Tezuka, M.; Kanchanapee, P.;
Natori, S. Chem. Pharm. Bull. 1971, 19, 2271. (e) Borsub,
L.; Thebtaranonth, Y.; Ruchirawat, S.; Sadavongvivad, C.
Tetrahedron Lett. 1976, 105.
(6) (a) Daengsvang, S.; Mangalasmaya, M. Ann. Trop. Med.
Parasitol. 1941, 35, 43. (b) Sadun, E. H.; Vajrasthira, S. J.
Parasitol. 1954, 40, 49.
(7) (a) Manfredi, K. P.; Blunt, J. W.; Cardellina, J. H. II;
McMahon, J. B.; Pannell, L. L.; Cragg, G. M.; Boyd, M. R.
J. Med. Chem. 1991, 34, 3402. (b) Hallock, Y. F.; Manfredi,
K. P.; Dai, J.-R.; Cardellina, J. H. II; Gulakowski, R. J.;
McMahon, J. B.; Schäffer, M.; Stahl, M.; Gulden, K.-P.;
Bringmann, G.; François, G.; Boyd, M. R. J. Nat. Prod.
1997, 60, 677.
To a solution t-BuLi (1.7 M, 30 mL, 50 mmol) and TMEDA
(7.5 mL, 50 mmol) in dry THF (100 mL) was slowly added
a solution of 2,2¢-N,N-diethylcarbamoyl-1,1¢-biphenyl (10,
7.68 g, 20 mmol) in THF (50 mL) at –78 °C under N2
atmosphere. The stirred reaction mixture was allowed to
attain r.t. overnight and treated with a sat. NH4Cl solution.
The organic solvent was removed in vacuo and the
remaining solution was extracted with CH2Cl2. The
combined organic layer was washed with H2O, brine, dried
(anhyd Na2SO4), and evaporated to give a viscous oil. After
purification by flash column chromatography using EtOAc–
hexane as eluent, 2,2¢-dihydroxybiphenyl-3,3¢-dicarboxylic
acid bisdiethyl amide(4) was obtained (6.14 g, 80%) as a
white solid; mp 140–141 °C (EtOAc–hexane). IR (KBr):
3428, 2981, 1600, 1572, 1488, 1450, 1408, 1353, 1311,
1259, 1141 cm–1. 1H NMR (200 MHz, CDCl3): d = 1.27 (t,
J = 7.0 Hz, 12 H), 3.53 (q, J = 7.0 Hz, 8 H), 6.99 (t, J = 7.2,
7.8 Hz, 2 H), 7.31 (dd, J = 1.8, 7.8 Hz, 2 H), 7.38 (dd,
J = 2.0, 7.7 Hz, 2 H). 13C NMR (50 MHz, CDCl3): d = 13.4,
41.2, 119.3, 120.8, 127.1, 127.2, 133.7, 149.0, 171.0. MS
(EI): m/z (%) = 385 (20) [M+ + 1], 384 (72) [M+], 383 (51),
313 (78), 312 (46), 311 (82), 310 (45), 295 (21), 285 (72),
283 (38), 240 (27), 239 (100). HRMS (FAB): m/z calcd for
C22H28N2O4 [M + H+]: 385.2127; found: 385.2128.
(15) Yield of 3b was improved to 53% using CuBr·SMe2.
(16) For intramolecular cyclization of allylbenzamide
derivatives, see: (a) Yu, S.; Rabalakos, C.; Mitchell, W. D.;
Wulff, W. D. Org. Lett. 2005, 7, 367. (b) de Koning, C. B.;
Michael, J. P.; Rousseau, A. L. J. Chem. Soc., Perkin Trans.
1 2000, 787. (c) de Koning, C. B.; Michael, J. P.; Rousseau,
A. L. Tetrahedron Lett. 1997, 38, 893. (d) Sibi, M. P.;
Dankwardt, J. W.; Snieckus, V. J. Org. Chem. 1986, 51,
271. (e) Hattori, T.; Takeda, A.; Suzuki, K.; Koike, N.;
Koshiishi, E.; Miyano, S. J. Chem. Soc., Perkin Trans. 1
1998, 3661. (f) Namsaaid, A.; Ruchirawat, S. Org. Lett.
2002, 4, 2633.
(8) Boyd, M. R.; Hallock, Y. F.; Cardellina, J. H. II; Manfredi,
K. P.; Blunt, J. W.; McMahon, J. B.; Buckheit, R. W. Jr.;
Bringmann, G.; Schäffer, M.; Cragg, G. M.; Thomas, D. W.;
Jato, J. G. J. Med. Chem. 1994, 37, 1740.
(9) (a) Sahakitpichan, P.; Thasana, N.; Ruchirawat, S. Synthesis
2005, 2934. (b) Govindachari, T. R.; Viswanathan, N.;
Ravindranath, K. R.; Anjaneyulu, B. Indian J. Chem. 1973,
11, 1081. (c) Mahidol, C.; Tarnchompoo, B.;
(17) Representative Procedure for the Double Ring Closure.
To a stirred THF (5 mL) solution of 2,2¢-dimethoxy-4,4¢-
bis(2-methylallyl)biphenyl-3,3¢-dicarboxylic acid bisdiethyl
amide (3a, 0.1316 g, 0.25 mmol) at –78 °C under Ar was
added a solution of MeLi (1.4 M, 1.1 mL, 1.5 mmol) in Et2O.
The solution turned deep violet and was allowed to warm to
r.t. and stirred at this temperature overnight. The reaction
was quenched by the addition of 20 mL of sat. NH4Cl and
extracted with CH2Cl2. The organic layer was combined,
washed with H2O, brine and dried (anhyd Na2SO4). The
crude product obtained after evaporation of CH2Cl2 was
purified by PLC using CH2Cl2–hexane (2:1) as eluent to give
white solid as 1,1¢-dimethoxy-6,6¢-dimethyl-2,2¢-binaph-
thalenyl-8,8-diol (1a, 0.0699 g, 75%); mp 234–236 °C
(EtOAc–hexane). IR (KBr): 3321, 2926, 1637, 1573, 1460,
1378, 1354, 1058 cm–1. 1H NMR (400 MHz, CDCl3):
d = 2.47 (s, 6 H), 3.58 (s, 6 H), 6.82 (d, J = 1.4 Hz, 2 H), 7.17
(br s, 2 H), 7.50 (d, J = 8.5 Hz, 2 H), 7.58 (d, J = 8.5 Hz, 2
H). 13C NMR (100 MHz, CDCl3): d = 21.7, 61.8, 112.9,
115.5, 118.4, 123.7, 124.3, 129.1, 136.5, 138.7, 153.4,
154.1. MS (EI): m/z (%) = 374 (61) [M+], 356 (15), 343 (23),
342 (81), 329 (36), 328 (100). HRMS (micro-TOF, ESI):
m/z calcd for C24H22O4 [M + H+]: 375.1591; found:
375.1584.
Thebtaranonth, C.; Thebtaranonth, Y. Tetrahedron Lett.
1989, 30, 3861.
(10) For the synthesis of naphthol derivatives via regiospecific
aryne annulation, see: (a) Hoye, T. R.; Chen, M.; Mi, L.;
Priest, O. P. Tetrahedron Lett. 1994, 35, 8747. (b) Hoye, T.
R.; Mi, L. Tetrahedron Lett. 1996, 37, 3097. (c) Hoye, T.
R.; Mi, L. J. Org. Chem. 1997, 62, 8586. (d) Hoye, T. R.;
Chen, M.; Hoang, B.; Mi, L.; Priest, O. P. J. Org. Chem.
1999, 64, 7184.
(11) (a) Beak, P.; Brown, R. A. J. Org. Chem. 1977, 42, 1823.
(b) de Silva, S. O.; Reed, J. N.; Snieckus, V. Tetrahedron
Lett. 1978, 5099. (c) Beak, P.; Brown, R. A. J. Org. Chem.
1979, 44, 4463. (d) Meyers, A. I.; Lutomski, K. J. Org.
Chem. 1979, 44, 4464. (e) Sibi, M. P.; Jalil Miah, M. A.;
Snieckus, V. J. Org. Chem. 1984, 49, 737.
(12) (a) Sibi, M. P.; Snieckus, V. J. Org. Chem. 1983, 48, 1935.
(b) Sibi, M. P.; Chattopadhyay, S.; Dankwardt, J. W.;
Snieckus, V. J. Am. Chem. Soc. 1985, 107, 6312.
(c) Parsons, A. S.; Garcia, J. M.; Snieckus, V. Tetrahedron
Lett. 1994, 35, 7537. (d) Blakemore, P. R.; Kilner, C.;
Milicevic, S. D. J. Org. Chem. 2005, 70, 373.
(13) (a) Casas, R.; Cavé, C.; d’Angelo, J. Tetrahedron Lett. 1995,
36, 1039. (b) Superchi, S.; Minutolo, F.; Pini, D.; Salvadori,
P. J. Org. Chem. 1996, 61, 3183. (c) Fürstner, A.; Seidel,
G.; Kindler, N. Tetrahedron 1999, 55, 8215.
(18) (a) Charette, A. B.; Chua, P. Synlett 1998, 163.
(b) Charette, A. B.; Grenon, M. Tetrahedron Lett. 2000, 41,
1677.
Synlett 2006, No. 7, 1080–1084 © Thieme Stuttgart · New York

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N. Thasana et al.
LETTER
(19) Representative Procedure for the Aryne Annulation.
A solution of lithium 2,2,6,6-tetramethylpiperidine (LTMP,
6.4 mmol) was prepared at 0 °C from 2,2,6,6-tetramethyl-
piperidine (1.1 mL) and n-BuLi (4.95 mL) in dry THF (20
mL). After 30 min, the solution was cooled to –78 °C and a
solution of N,N-diethyl seneciomide (7a, 0.66 g, 4.3 mmol)
in dry THF (5 mL) was added and stirred at this temperature
for 1 h. A solution of tetrabromo-2,2¢-dimethoxybiphenyl
(6, 0.57 g, 1.1 mmol) in dry THF (15 mL) was added drop-
wise, the reaction turned to dark red. After the addition was
complete, the reaction was slowly warmed to r.t. and stirred
overnight. Then, sat. NH4Cl was added and the mixture was
extracted with CH2Cl2. The combined organic layer was
washed with H2O, brine and dried (Na2SO4) and evaporated
to dryness. Further purification was carried out by PLC
(SiO2, 4% EtOAc–hexane) to obtain binaphthol 12 (0.117 g,
20%).
Compound 12: mp 249–250 °C (EtOAc–hexane). IR
(CHCl3): 3344 (OH), 3010, 1636, 1369, 803 cm–1. 1H NMR
(200 MHz, CDCl3): d = 2.55 (s, 6 H), 3.61 (s, 6 H), 6.91 (d,
J = 1.3 Hz, 2 H), 7.56 (d, J = 0.9 Hz, 2 H), 7.84 (s, 2 H), 9.76
(s, 2 H). 13C NMR (50 MHz, CDCl3): d = 22.0, 62.3, 114.3,
116.3, 118.2, 118.3, 123.1, 132.1, 134.4, 140.4, 153.4,
154.4. MS (EI): m/z (%) = 534 (50) [M+ + 2], 532 (100) [M+],
530 (48), 488 (41), 486 (84), 484 (42). Anal. Calcd for
C24H20Br2O4: C, 54.16; H, 3.79. Found: C, 54.27; H, 3.76.
(20) Adimurthy, S.; Ramachandraiah, G. Tetrahedron Lett. 2004,
45, 5251.
(21) Bringmann, G.; Ortmann, T.; Feineis, D.; Peters, E.-M.;
Peters, K. Synthesis 2000, 383.
Synlett 2006, No. 7, 1080–1084 © Thieme Stuttgart · New York